The present invention relates to a process for homogenizing the winding hardnesses of a roll profile of a film reel, in which a frame feed-in control for the film web is driven following the film edges. The invention also relates to a device.
Despite highly developed closed-loop control technology for adjusting the die gap of slit dies, extruded films have so-called permanences. These permanences are thin/thick points which are distributed over the width profile of the film and can be encountered for long periods of time always at the same points on the film-width profile. These permanences are the consequence of the finite number of adjustment screws in the upper die lip for adjusting the die gap. These permanences can be assigned to the distances between these adjustment screws, which are generally approximately 25 mm. These permanences, which are first to be encountered in the prefilm, are also retained during the longitudinal and transverse stretching of the prefilm to form the final film. During winding of the final film to form a film reel, these thin and thick points in the film reel accumulate and lead to varying winding hardnesses across the roll width of the film reel. In the case of relatively long storage times of the film reels or film rolls, these varying winding hardnesses in turn lead to irreparable straining of the film, so that the film may become unusable.
To avoid the adverse effects of these permanences, it is known to traverse the winding core of the film reel during winding, that is to say, to move it back and forth across the width of the film web. The mechanics and control for this traversing of the winding core are very complicated.
Other measures for preventing these permanences consist in introducing the longitudinally stretched prefilm in an oscillating manner into the transverse stretching frame to achieve an oscillation of the thin/thick points transversely to the film travel direction and thus to distribute the permanences uniformly across the width of the film reel without accumulating them at specific points. The oscillating introduction of the longitudinally stretched prefilm into the transverse stretching frame may be achieved by various measures. One of these is a targeted movement of a roll, for example, a deviation from the horizontal, upstream of the transverse stretching frame, with simultaneous stoppage of the frame feed-in control. Another measure consists in carrying out a selective prefilm trimming, in which broad edge strips are cut off alternately on both sides of the film web, depending on the run length, while maintaining a constant width of the trimmed prefilm. The displacement of the film edges by the traversing trimming is followed by the frame feed-in control, and as a result an oscillating transverse displacement of the thin/thick points of the prefilm results.